Despite their seemingly disparate looks, natural settings include some statistical regularities (e.g., mountains, rainforests, deserts). First, they have a distinct distribution of luminance intensities that decrease with spatial frequency, known as the 1/f amplitude spectrum. Furthermore, natural sceneries have constant geometric qualities, such as identical densities of structure across many sizes, which classifies them as fractal (e.g., how the branching patterns of rivers and trees appear similar irrespective of scale). These two qualities are inextricably linked and significantly correlate in natural landscapes. However, research utilizing thresholded noise pictures reveals that the human visual system is preferentially tuned to natural scene structure over 1/f spectra in spatial tuning. It was unknown if the reliance on natural geometry extends to the time realm.
The discriminating sensitivity to two types of synthetic noise movies: grayscale and thresholded (N = 60), was measured using a psychophysics test. Each movie type has similar geometric qualities (fractal D) but significantly different spectrum properties (measured). Researchers saw a consistent reliance on stimulus structure across movie types in space and time, with sensitivity peaking for natural geometry stimuli despite having different 1/f spectra. The findings suggested that the brain mechanisms underpinning the tuning have evolved to be sensitive to the most stable signal in the natural environment—structure—even though they were only tested behaviorally (e.g., the structural properties of a tree are consistent from morning to night despite illumination changes across time points).